So the loss in the sum = 40x5.Now this loss of 40x5 must be compensated by our second group i.e. 30 students.So their average must be not only the initial 80, but also the average meant to compensate the loss incurred because of the first group.Hence the average of the remaining 30 students = 80 + (40x5)/30 = 80 + 20/3 = 86.66I hope this is lucid. ________